Tomoyoshi Motohiro

Enhanced Conversion Efficiencies of Cu2ZnSnS4-Based Thin Film Solar Cells by Using Preferential Etching Technique

Cu2ZnSnS4 (CZTS) thin film solar cells have been fabricated by co-sputtering technique using three targets of Cu, SnS, and ZnS. CZTS-based thin film solar cells over 6.7% efficiency were obtained for the first time by soaking the CZTS layer on the Mo coated soda-lime glass substrate in deionized water (DIW) after forming the CZTS layer. It was found that DIW-soaking had the effect of preferential etching, which eliminated selectively metal oxide particles in the CZTS layer, by electron probe X-ray micro analysis.

Thin film retardation plate by oblique deposition.

The birefringent property of obliquely deposited metal oxides was studied with a view to applying it to optical retardation plates. By finding favorable conditions to form transparent films of large retardation and low opacity, we developed homogeneous quarterwave plates with a bilayered structure 60 x 250 mm in size and ~3 microm thick on glass substrates. These retardation plates can work with a normally incident light based on form birefringence caused by the characteristic anisotropic microstructure inside the film. They showed promising optical properties which can compete with the conventional types of retardation plate.

Photoelectrochemical reduction of CO2 in water under visible-light irradiation by a p-type InP photocathode modified with an electropolymerized ruthenium complex

Photoelectrochemical reduction of CO(2) to HCOO(-) was successfully achieved by a p-type InP photocathode modified with an electropolymerized ruthenium complex in water. This technique decreased the required applied potential for CO(2) reduction by utilizing solar energy. The carbon and proton sources of HCOO(-) were identified by a tracer experiment to be CO(2) and H(2)O, respectively.

Cu2Sn1-xGexS3 (x = 0.17) Thin-Film Solar Cells with High Conversion Efficiency of 6.0%

We have fabricated Cu2Sn1-xGexS3 thin-film solar cells by cosputtering deposition of Cu and Sn followed by sulfurization in S and GeS2 vapors. The conversion efficiency was significantly improved to be as high as 6.0% compared with the values of Cu2SnS3 solar cells similarly fabricated. Scanning electron microscopy observation revealed that alloying with Ge accelerated the grain growth during the sulfurization process, contributing to the improvement in the conversion efficiency. The bandgap energy of Cu2Sn0.83Ge0.17S3 was about 1.0 eV, which is suitable for bottom cells used in double-junction solar cells.

Monte Carlo simulation of the particle transport process in sputter deposition

Abstract The essential features of the transport of sputtered particles from a target to a substrate during sputter deposition were studied by calculation using the Monte Carlo technique. The study takes into consideration the change in momentum as well as the kinetic energy loss of sputtered particles in their collisions with ambient gas molecules, to gain an understanding of the effects of these factors and of the number of sputtered particles arriving at a substrate on the mechanism of growth of a thin film by sputter deposition. Some theoretical predictions using the above calculation were made for several selected conditions of sputter deposition.

6% Efficiency Cu2ZnSnS4-based thin film solar cells using oxide precursors by open atmosphere type CVD

An open atmosphere type chemical vapor deposition (OA-CVD) method is one of the most effective methods for producing functional thin films. Especially, the OA-CVD method is a unique technique which is able to deposit metal oxide thin films by decomposition of vaporized raw materials released through a nozzle onto substrates in the air. Cu2ZnSnS4 (CZTS)-based thin films as absorber layers of thin film solar cells were fabricated by sulfurizing oxide precursor thin films synthesized by the OA-CVD method. Cu(C5H7O2)2, Zn(C5H7O2)2 and Sn(C5H7O2)2 were used as raw materials. The oxide precursor thin films were sulfurized at 520–560 °C in 5 vol% H2S balanced with N2. The formed CZTS-based thin films included oxygen with the composition ratio of O/(S + O) = 0.17–0.27 according to energy dispersive X-ray spectroscopy. The thin film solar cells using the CZTS-based thin films including oxygen [CZT(S,O) films] as the absorber layers were fabricated. The CZT(S,O) thin film solar cell had a stack structure of Al/Al-doped-ZnO/CdS/CZT(S,O)/Mo/soda-lime glass substrate. The efficiency of the CZT(S,O) thin film solar cells was 6.03%, which was the high efficiency in the reported value for CZTS-based thin film solar cells using oxide thin film precursors. It was found that the OA-CVD method is suited to fabricate the absorber layers of thin film solar cells.

Hot carrier solar cells operating under practical conditions

We theoretically investigated the features of hot carrier solar cells, from which photogenerated carriers are extracted before they are completely thermalized. There are three channels of energy dissipation from photogenerated carriers that lowers the conversion efficiency: thermalization in the absorber, emission from the absorber, and thermodynamically unavoidable heat flux to the ambient. The emission increases with increasing carrier density in the absorber, whereas the heat flux decreases. Previous calculations of the conversion efficiency have been carried out under the supposition of no thermalization of carriers. In this case, the dominant process of energy dissipation is the emission, like conventional solar cells represented by the Shockley and Queisser formula. In practice, the carriers should be extracted to external circuits immediately after photogeneration because they are partially thermalized. This restriction leads to a much smaller carrier density and consequently more significant energ...

Analysis of lattice site occupancy in kesterite structure of Cu2ZnSnS4 films using synchrotron radiation x-ray diffraction

The relationship between the composition and the kesterite crystal structure of Cu2ZnSnS4 (CZTS) thin films was investigated. CZTS thin films with three different compositions were produced by sulfurizing the precursors in 5 vol. % H2S balanced with N2. We measured the x-ray diffraction patterns of the CZTS powders scraped away from the CZTS thin films using the synchrotron radiation. The site occupancy of the CZTS thin films that had respectively different compositions were determined by the Rietveld analysis using x-ray powder diffraction patterns. Zn substitutes for Cu at the 2a site with decreasing the value of composition ratio Cu/(Zn+Sn). Meanwhile, Cu substitutes for Sn at the 2b site with increasing the value of Cu/(Zn+Sn). The CZTS thin film with Cu/(Zn+Sn) = 0.8 especially shows a marked tendency that Cu substitutes for Zn at the 2d site and/or Zn substitutes for Cu at the 2c site compared to the CZTS thin films with Cu/(Zn+Sn) = 1.0 and 1.2. We suppose that it is very important to realize the h...

Dual functional modification by N doping of Ta2O5: p-type conduction in visible-light-activated N-doped Ta2O5

We report dual functional modulation, both p-type conduction and band gap narrowing, of Ta2O5 semiconductor induced by heavy doping of nitrogen in films sputtered in N2/Ar mixture and ammonia-treated powders. The N doping induced a redshift in the optical absorption edge from 320 to 500 nm, resulting in the absorption of visible light. Simultaneously, the N doping caused a change in the conduction from n-type to p-type. As a result, the N–Ta2O5 photoelectrode containing 7.6 or 16.1 at. % of N exhibited a distinct cathodic photocurrent (due to p-type conduction) in solutions under visible light irradiation (>410 nm).

Ionization potentials of transparent conductive indium tin oxide films covered with a single layer of fluorine-doped tin oxide nanoparticles grown by spray pyrolysis deposition

Indium tin oxide (ITO) films deposited with single layers of monodispersive fluorine-doped tin oxide (FTO) nanoparticles of several nanometers in size were grown on glass substrates by intermittent spray pyrolysis deposition using conventional atomizers. These films have significantly higher ionization potentials than the bare ITO and FTO films grown using the same technique. The ITO films covered with FTO particles of 7nm in average size show an ionization potential of 5.01eV, as compared with ∼4.76 and ∼4.64eV in ITO and FTO films, respectively, which decreases as the FTO particle size increases. The ionization potentials are practically invariant against oxidation and reduction treatments, promising a wide application of the films to transparent conducting oxide electrodes in organic electroluminescent devices and light-emitting devices of high efficiencies.